One example of a disk device is a magnetic disk device that includes a magnetic disk disposed within a case as a recording medium, a spindle motor which supports and rotationally drives the magnetic disk, and a magnetic head which reads and writes information from and in the magnetic disk. The magnetic head is supported by a rotatable head actuator, moved along the radial direction of the magnetic disk, and positioned there.
Recently, magnetic recording heads according to a perpendicular magnetic recording and thermally-assisted recording have been proposed in order to increase recording density and capacity of a magnetic disk device and reduce the size thereof. This magnetic head includes a near field light generating element which generates a near field light toward a recording layer of the recording medium and a waveguide for propagating a laser light for generating the near field light. In such a magnetic recording head, when writing information, a laser light generated from a laser generating element and passing through the waveguide generates surface plasmon outside the waveguide and this surface plasmon excites plasmon in the near field light generating element. According to this, the recording layer of the recording medium is irradiated with the near field light from a distal end of the near field light generating element and locally heated, so that a coercive force is sufficiently reduced in the recording layer portion, which enables a high recording density.
In the magnetic recording head, it is difficult to obtain a high energy propagation efficiency, which is the ratio of energy propagated to the near field light generating element to the laser light energy. When the propagation efficiency is low, a high current has to be applied to the laser generating element to generate a laser light of high power in order to obtain a near field light of sufficient intensity for the thermally-assisted recording. Therefore, power consumption increases in the magnetic disk device. Further, the laser light not propagated is radiated from a terminal end of the waveguide, which may cause an unexpected heating of the recording medium and erasing of the recorded data, and also deterioration or destruction of an element caused by the heating of the waveguide.